The somatomedins comprise a group of growth hormone-dependent, insulin-like peptides with anabolic and mitogenic activity for a wide variety of cell lines. The two sequenced human somatomedins, insulin-like growth factors I and II (IGF-I and II) are structurally homologous to human proinsulin, suggesting concservation of a core peptide skeleton which has evolved into a family of hormones capable of responding to both metabolic and mitogenic cellular requirements. The actions of insulin and the somatomedins are presumably mediated through the binding of these peptides to specific membrane receptors on target tissues, but attempts to define the mechanism of action of each peptide have been handicapped by the significant structural and functional homology of their receptors. This project will attempt to characterize these receptors by employing a panel of polyclonal and monoclonal antibodies generated against receptor preparations at various levels of purification. Antibodies will be produced which are specific for the insulin and the IGF-I receptor, and which are directed against various protions of each receptor, including 1) the ligand binding site, 2) antigenic sties removed from the binding site, and 3) the "effector" protion of the receptor. The techniques of immunoprecipitation of solubilized receptors, competitive radioligand binding studies, and affinity cross-linking and electrophoresis will be employed to study 1) tissue and species specificity of each receptor, 2) the ontogeny of each receptor, 3) alterations in receptors associated with cell differentiation, and 4) mechanism of modulation of each receptor by homologous and heterologous ligands. Antibodies which are specifically directed against each receptor will be employed to define the respective role of the insulin receptor, IGF-I receptor and IGF-II receptor in cell metabolism and replication. Finally, anti-receptor antibodies will be administered to both normal and hypophysectomized rats, and growth parameters will be measured after treatment with growth hormone, insulin and IGF-I. These studies will help define the specific roles of insulin and the somatomedins in cell growth, and will provide the first direct assessment of the fundamental somatomedin hypothesis, that peptides mediate the anabolic actions of growth hormone.